Carnivorous plant inspires super-slippery material

Water, oil, blood and insects alike slide swiftly off a new super-slippery material inspired by a carnivorous plant.

Scientists searching for clever materials sometimes borrow ideas from nature. Lotus leaves, for example, are famously water repellant, thanks to their textured surfaces which trap a cushion of air for water to slide down.

The leaves have inspired a range of so-called "superhydrophobic" materials. But these materials have trouble repelling oils and more complex liquids, which have lower surface tensions than water and can seep into the surface at the slightest pressure.

"Everybody's talking about the lotus leaf," says Joanna Aizenberg of Harvard University, whose lab designs and builds biomimetic materials to solve a range of problems. "But it's just one of many strategies that nature created to manage and control the interaction with liquid."

Now, a new material takes a cue from one of the plant world's few meat-eaters: the carnivorous pitcher plant Nepenthes. The plants prey on insects, whose oily feet normally allow them to walk up walls. But pitchers' tube-shaped leaves have microscopic bumps that hold a thin layer of water in place. The water repels the oils, sending hapless insects slipping straight into their gaping mouths.

"They just step on the rim, and immediately slide into the digestive juices," Aizenberg says.

Aizenberg realized that with the right choice of lubricating liquid, the pitcher plant's strategy could be adapted to repel virtually anything.

The researchers started with a textured substrate, which could be almost anything that is rough on the nanoscale, Aizenberg says. One good choice is Teflon, a fibrous material that is widely thought to be super-slippery itself.

Filling in the bumps

Their most slippery surface resulted when they added a layer of the perfluorinated fluid 3M Fluorinert FC-70, manufactured by the firm 3M, to Teflon. The liquid oozed into all the pores in the Teflon, and left a nanometres-thin layer of liquid at the top. The material still feels dry to the touch, and other liquids simply hydroplane off the surface, like a car sliding off a wet road. The team calls the material 'slippery liquid-infused porous surfaces,' or SLIPS.

"We call it SLIPS, because everything does," Aizenberg says. The materials could be useful for making self-cleaning windows, friction-free oil and water transport pipes, and safe and efficient blood transfusion devices, she adds.

SLIPS beat the lotus leaf in several arenas. They're more slippery – liquids from water to oil to blood lose contact with the surface when it's tilted by an angle as shallow as 2 degrees, whereas liquids held to other surfaces tilted from 5 to 30 degrees. They can also recover from damage, because the lubricating liquid naturally seeps back in to any holes. And because liquid is incompressible, the material can be used at pressures equivalent to 7 kilometres underwater.

"It's interesting that it combines self-lubrication, self-healing and self-cleaning, which are different processes," says Michael Nosonovsky of the University of Wisconsin-Milwaukee. "It's a new type of smart material."

Journal reference: Nature, DOI: 10.1038/nature10447

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